Communication devices such as User Equipments (UEs) are also known as e.g. terminals, mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or Pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
D2D Communication
In cellular network assisted Device-to-Device (D2D) communications, also referred to as D2D communications as a cellular underlay, user equipments in the vicinity of each other typically less than a few 10 s of meters but sometimes up to a few hundred meters, can establish a direct radio link, also referred to as a D2D bearer. While user equipments communicate direct over the D2D bearer, they also maintain a cellular connection with their respective serving base station. In this way the cellular RAN can assist and supervise the user equipments in allocating time, frequency and code resources for the D2D bearer. Also, the cellular RAN controls mode selection, meaning that the cellular RAN decides whether the D2D pair should use the direct link or communication should take place via the base station. The RAN also sets the maximum power level that the D2D pair may use for the D2D bearer.
Thus the basic rationale for network assisted D2D communications is to take advantage of the short distances between user equipments, reuse cellular spectrum and at the same time to protect the cellular layer from potentially harmful interference caused by the D2D bearer.
Sounding Reference Signals and Demodulation Reference Signals in LTE Advanced Systems
In LTE and LTE-Advanced systems, reference signals are designed to aid channel estimation at a receiver. Obtaining information about a wireless channel is useful for different purposes such as allowing coherent demodulation of transmitted symbols and making frequency channel dependent scheduling decisions. In the LTE uplink, two Reference signals are defined, the DeModulation Reference Signal (DMRS) and the Sounding Reference Signal (SRS).
The DMRS in LTE is associated with both the Physical Uplink Shared Channel (PUSCH) also referred to as the data channel, and the Physical Uplink Control Channel (PUCCH) also referred to as the control channel. This is to facilitate the coherent demodulation of both data and control signals in the resource blocks that the user equipment is actually transmitting. The SRS is transmitted by the user equipment within a bandwidth that is greater than the currently allocated bandwidth for this user equipment. In this way the base station has knowledge about channels that are currently not used by the user equipment which is useful for subsequent scheduling decisions at the base station.
In general, the dynamic estimation of the channel regarding frequency response, at the receiver is typically aided by such reference signals, sometimes also called pilots or pilot signals. The basic idea of using such pilot signals is to rely on transmitted symbols that are known at the receiver.
Interference Suppression and Interference Cancellation by LTE Advanced UEs
A communication link in a cellular communications network may be subject to different types of interference. First, the multipath environment results in distortion to the transmitted signal, giving rise to intersymbol interference. Intersymbol interference is a form of distortion of a signal in which one symbol interferes with subsequent symbols. Second, spatial multiplexing such as single-user or multi-user Multiple Input Multiple Output (MIMO) introduces spatial-multiplexing interference. Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals from each of the multiple transmit antennas. Third, due to frequency reuse, intercell interference may have an impact on link performance. Advanced receivers exploit interference signal structure and statistics to mitigate or reduce performance degradation.
For example, a linear Minimum-Mean-Squared-Error (MMSE) or Interference Rejection Combiner (IRC) receiver can alleviate interference by exploiting the spatial and/or temporal correlations in an interference signal. This may be thought of as a form of receiver beamforming, using the receiver degree of freedom, i.e. number of receive antennas, number of fingers in the case of CDMA system, etc., to suppress interference. It does not need much information about the interfering signal. In this type of receiver, interference spatial and temporal correlations may be estimated blindly, i.e. without knowing anything about the interfering signal. However, it is very helpful if the receiver knows the time interval in which the interference characteristics stay approximately the same, so that it can obtain and update interference correlations accordingly.
Yet, a more significant performance improvement may be achieved by an advanced receiver that fully exploits the interference signal structure. For example, if the transport format, e.g. modulation format and coding scheme, and reference signal, e.g. pilot symbols, see the previous section on reference/pilot signals, used by an interfering signal is known to the receiver, the receiver may attempt to detect the interfering signal and then cancel it based on the detected signal. Examples for this kind of receiver include post-decoding Successive Interference Cancellation (SIC) receiver, post-decoding Parallel Interference Cancellation (PIC) receiver, and Turbo Interference Cancellation (Turbo-IC) receiver, etc. These interference cancellation receivers may also include MMSE, e.g. MMSE-SIC, MMSE-PIC, etc.
Interference cancellation by these advanced receiver structures may lead to the elimination of the interference, which in some cases the interference is completely cancelled, whereas in other cases the impact of interference on the useful signal is reduced.
A problem may be exemplified in the following scenario, in which it is assumed that D2D communication uses UL cellular resources, i.e. UL spectrum in an Frequency-division duplexing (FDD) system or UL time slots in a Time-Division Duplex (TDD) system. FDD, TDD and half duplex FDD (HD-FDD) and different types of duplex techniques use in wireless communication systems. Using FDD means that the transmitter and receiver operate at different carrier frequencies. Well known examples of FDD system are WCDMA (aka UTRA FDD), LTE FDD etc. In TDD the transmitter and receiver operate on the same carrier frequency but in different time instances (e.g. time slot, subframe). Well known examples of FDD system are UTRA TDD, LTE TDD etc. The HD-FDD is a special case of FDD (aka as full duplex FDD) and in which case the transmitter and receiver operate on different carrier frequencies like in FDD but in different time instances (e.g. time slot, subframe) like in TDD. A well known example of HD-FDD system is GSM. A D2D receiving user equipment is the victim of interference from cellular user equipments and from other D2D transmitting user equipments. The cellular UEs may themselves be D2D capable user equipments or legacy user equipments without D2D capability.
The D2D receiving user equipment may employ different types of receiver algorithms to mitigate, e.g. suppress, cancel or eliminate, interference, as described above.
Interference management by Radio Resource Management (RRM) methods in a mixed cellular and D2D environment may be used. For example the measurements performed by the D2D UE and/or cellular UE on other D2D UE may be used by the network to control interference in the network. For example the network may assign resources to fewer D2D UE when interference based on reported measurement results is higher than a threshold. An example of this is shown in WO 2011/124015.
However, currently, the received interference cannot be adequately managed by the D2D user equipment. This results in that the received signal quality at the D2D user equipment is deteriorated.